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Not All Seaweeds Are Superfoods

Only some seaweeds pack iron your body can actually use.

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Among health-conscious eaters, seaweed holds a reputation as a nutrient-rich superfood. Seaweed is stuffed with vitamins and protein, chock full of iron—and at least one kind tastes like bacon. If that’s not a superfood, we don’t know what is.

Seaweed, however, has the same problem as other foods, super or not: even if packed with nutrients, there’s no guarantee your body can access each and every nutritional goodie offered. And as new research shows, some seaweeds are more healthful than others—at least when it comes to iron.

In their lab at the University of Hawaii, nutritionists studied 13 varieties of popular edible seaweeds, and found that, while many of them are indeed rich in iron, only two types—nori and sea lettuce—provide more bioavailable iron than you’d get from eating the same amount of spinach. Popeye would be pleased.

But the researchers also found that several types of seaweed contain high levels of arsenic and other potentially harmful minerals, making them risky to consume in large quantities. In particular, hijiki, a popular type of seaweed that is often consumed for its iron content, tested high in arsenic, while a sample of red ogo contained excessive amounts of manganese. The prevalence of toxins varied according to where the seaweed had been grown. That means it may not be a healthy idea to double- or triple-down on eating seaweed as a “superfood.”

The research reinforces what food experts have been saying all along, says Joannie Dobbs, a clinical nutritionist who worked on the study. “Ideally we get our nutrients from a variety of foods, obtained from a variety of locations—and remember: there are no superfoods. These principles include seaweed.”

Dobbs says researchers started studying the bioavailability of iron in seaweed because many people—especially vegetarians and those who consume very little red meat—have iron deficiencies. So scientists set out to identify alternative, plant-based sources of iron.

By breaking down samples of the various seaweeds using digestive fluids, and then “feeding” the solution to human intestinal cells in the lab, the scientists tested iron absorption rates. Michael Dunn, a researcher who worked on the project, says he was surprised by the nutritional variability of the seaweed. He was also surprised to find that adding vitamin C to the mix further improved the intestinal cells’ absorption of the iron from nori and sea lettuce.

“Only some seaweeds can be considered good sources of bioavailable iron,” says Dunn. If you’re looking for the most iron-rich seaweed for your sushi or smoothie, then nori or sea lettuce are the ways to go. And for the servers out there, we apologize: soon you’ll need to know the seaweed’s species, as well as the chicken’s name.

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Is Seaweed a Superfood?

Seaweed is a form of marine plants and algae that grow in salty water. Traditionally an Asian-cuisine favorite, this sea vegetable has made its way across the oceans and become a popular ingredient in western dishes, too — you’ll see it wrapped around sushi, roasted as a kid’s snack, and served as a bright green seaweed salad. It’s unsurprising, as the delicious umami-flavored ingredient is not only versatile, but low in calories and packs a serious nutritional punch.

Humans have been foraging for seaweed for centuries, with some researchers suggesting we have seaweed to thank for our highly evolved brains. But due to potentially high arsenic and iodine levels, which come with scary cancer warnings, you need to consider your choice of seaweed and how much you’re eating.

Get the gist on this Asian staple ingredient.

Seaweed Delivers Plenty of Nutrients in Very Few Calories

By sprinkling some seaweed into your salad, you’ll be boosting your dose of disease-fighting nutrients, including essential amino acids, antioxidants, omega-3 fatty acids, vitamins, minerals, and dietary fiber. Take a look at a few of the health benefits:

  • Seaweed is an excellent source of iodine, a mineral vital during pregnancy and early childhood to ensure proper growth and brain development.
  • A rich source of fiber, seaweed may help you curb your appetite, reduce blood cholesterol, and control your blood sugar levels. The fiber also acts as a prebiotic or food for the good bacteria in your gut.
  • Wakame seaweed, the type traditionally added to miso soup, has been shown to boost immunity and have antiviral activity, particularly against the herpes virus which is responsible for giving you cold sores.
  • With its high amino acid, iron, and vitamin C content, seaweed is a nourishing addition to a vegan diet. Some varieties, like nori, also show promise in being a source of vitamin B12, however, it’s still not entirely clear whether the body can absorb vitamin B12 from seaweed.

Don’t Go Overboard

While the high levels of iodine have benefit for growing brains, consuming too much iodine can be harmful. Some kelps, a type of seaweed often consumed as a supplement, contains very high amounts of iodine, and eating too much can affect your thyroid function and possibly lead to thyroid cancer, although a recent large study on Japanse women found no association between eating natural seaweed (not taking a supplement) a few times a week and cancer risk. As iodine content can vary greatly and supplements deliver a large dose, it’s best to not go overboard on kelp..

Another type of seaweed, Hijiki, contains high levels of arsenic, a heavy metal which can act as a carcinogen in the body. For this reason, most food standards agencies around the world, including the USDA, have advised the public to avoid eating Hijiki seaweed. Hijiki is easy to spot because of its distinctive black and shredded appearance. It’s mainly added to Japanese and Korean soups and salads, but is not used to make sushi.

And don’t confuse minimally processed seaweed with its highly processed by-product, carrageenan a food additive used as a stabilizer and thickener in foods like non-dairy milks and ice-cream, which can cause tummy upsets in some people.

Seaweed is Another Way to Eat More Plants

If a superfood is something that’s packed with nutrients and comes with serious health benefits, then yes seaweed can be considered a superfood. Does this mean you should be snacking on copious amounts of seaweed or adding it to every salad? Nope. Like with all things in life, moderation is key. View seaweed as another tasty way to add more nourishing plants to your plate, along with other vegetables, fruit, whole grains, nuts, and legumes. The good news is because seaweed supplies a concentrated dose of nutrients, you only need a small amount to boost the nutritional quality of your meal. Go for natural seaweed over supplements to safely include it in small quantities, a couple of times a week as part of an overall healthy diet.

This information is for educational purposes only and is not intended as a substitute for medical diagnosis or treatment. You should not use this information to diagnose or treat a health problem or condition. Always check with your doctor before changing your diet, altering your sleep habits, taking supplements, or starting a new fitness routine.

Tracy Morris

Tracy Morris is Fitbit’s Lead Nutritionist. With a master’s degree in nutrition and dietetics from South Africa, she’s also an Australian Accredited Practising Dietitian, and an international member of the Academy of Nutrition and Dietetics in the US. Over the past 20 years, Tracy’s lived in five different countries, inspiring people around the globe to be healthy. She currently lives in sunny Sydney, Australia where she helps Fitbit fans around the globe live their best lives. When she’s not working, running after her three young kids, or practicing pilates, she can be found sipping pinot noir with her husband watching the sun set.

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This sea vegetable has fast become a popular ingredient across the world. Discover the health benefits and learn whether it’s possible to eat too much.